1. Field of the Invention
This invention relates to a PLL (phase locked loop) circuit which can be used with a synchronous detector circuit for detecting video signals in a television and more particularly to a PLL detector circuit which eliminates the need for adjustments to a resonator of a VCO (voltage controlled oscillator).
2. Description of the Related Art
Synchronous detection technology is indispensable to the communication and television fields. A PLL circuit is used to obtain continuous oscillation output synchronized with the frequency and phase of an input signal.
FIG. 1 shows an example of a conventional PLL circuit, wherein a phase comparator 101 consists of a multiplier and outputs a signal for the difference between two input signals. A loop filter 102 consists of a low pass filter and smooths the signal for the difference input from the phase comparator 101. A VCO (voltage controlled oscillator) 103 contains an LC oscillator, etc., for changing the oscillation frequency in response to the signal input from the loop filter 102. An input signal and a signal from the VCO 103 are input to the phase comparator 101. Thus, the PLL circuit operates so as to reduce the difference between the two signals input to the phase comparator 101; the output signal of the VCO 103 is synchronized with the input signal at the same frequency, and thus can be used for synchronous detection, etc.
FIG. 2 shows the characteristics of the PLL circuit, wherein the horizontal axis shows frequencies and the vertical axis shows output of the loop filter 102. Assume that f0 is a free running frequency of the VCO. The free running frequency refers to a frequency at which the VCO freely oscillates without being locked. Consider an input frequency that will be raised from the lower stage. First, the VCO 103 starts at a state in which it is not locked to input. At the f2 point, the loop enters a lock state. When the frequency is further raised, the loop is unlocked at the f4 point. Likewise, when the frequency is lowered from the upper stage, the loop enters a lock state at f3, then is unlocked at fl. The range between the lower and upper limits of frequencies, f2 and f3, where the loop not locked enters a lock state is referred to as a pull-in range. The range between the lower and upper limits of frequencies, fl and f4, where the loop locked is unlocked is referred to as a hold range. Normally, these ranges are symmetrical with the free running frequency f0 as the shared center point of the two ranges.
Next, consider a phase error between input and VCO output in a stationary state. When the input signal frequency equals the free running frequency of the VCO, the phase error is zero. However, if the free running frequency shifts, the phase comparator supplies a voltage, thus a finite stationary phase error occurs. Assuming that AF is the free running frequency shift, that Kd is the phase comparator gain, and that Ko is the VCO control sensitivity, the stationary phase error .DELTA..theta. is represented by the following expression: EQU .DELTA..theta.=.DELTA.F/(Kd.multidot.Ko) (1)
When the PLL circuit is used to make a synchronous detector for AM signal demodulation, the characteristics of detection output distortion, etc., deteriorate in proportion to the stationary phase error. Normally, LC tank circuits with large variations are often used to make VCOs. Therefore, in such systems, an adjustment is essential to minimize the VCO free running shift.
On the other hand, an LC resonator consisting of a coil and capacitor is often used as a VCO oscillator. However, since the LC resonator is low in Q (quality factor), oscillation frequency adjustment is required.
LC resonators must be adjusted one at a time on a manufacturing line of a factory and this adjustment step is cumbersome. The above-mentioned Q needs to be raised to accurately determine the center frequency; if it is raised, the pull-in range of the PLL circuit is narrowed, resulting in poor resistance to external noise. This is the only place in the PLL detector circuit block where an adjustment is necessary at present. A solution to this problem is desired.